A node class is a class that,
? relies on the base class for services and implementation,
? provides a wider interface to the users than its base class,
? relies primarily on virtual functions in its public interface
? depends on all its direct and indirect base class
? can be understood only in the context of the base class
? can be used as base for further derivation
? can be used to create objects.
A node class is a class that has added new services or functionality beyond the
services inherited from its base class.